drying foods freezing product

The term food preservation refers to any one of a number of techniques used to prevent food from spoiling. It includes methods such as canning, pickling, drying and freeze-drying, irradiation, pasteurization, smoking, and the addition of chemical additives. Food preservation has become an increasingly important component of the food industry as fewer people eat foods produced on their own lands, and as consumers expect to be able to purchase and consume foods that are "out of season."

Smoking

Early humans probably discovered by accident that certain foods exposed to smoke seem to last longer than those that are not. Meats, fish, fowl, and cheese were among such foods. It appears that compounds present in wood smoke have anti-microbial actions that prevent the growth of organisms that cause spoilage.

Today, the process of smoking has become a sophisticated method of food preservation with both hot and cold forms in use. Hot smoking is used primarily with fresh or frozen foods, while cold smoking is used most often with salted products. The most advantageous conditions for each kind of smoking—air velocity, relative humidity, length of exposure, and salt content, for example–are now generally understood and applied during the smoking process. For example, electrostatic precipitators can be employed to attract smoke particles and improve the penetration of the particles into meat or fish.

So many alternative forms of preservation are now available that smoking no longer holds the position of importance it once did with ancient peoples. More frequently the process is used to add interesting and distinctive flavors to foods.

Drying

Since most disease-causing organisms require a moist environment in which to survive and multiply, drying is a natural technique for preventing spoilage. Indeed, the act of simply leaving foods out in the sun and wind to dry out is probably one of the earliest forms of food preservation. Evidence for the drying of meats, fish, fruits, and vegetables go back to the earliest recorded human history.

At some point, humans also learned that the drying process could be hastened and improved by various mechanical techniques. For example, the Arabs learned early on that apricots could be preserved almost indefinitely by macerating them, boiling them, and then leaving them to dry on broad sheets. The product of this technique, quamaradeen, is still made by the same process in modern Muslim countries.

Today, a host of dehydrating techniques are known and used. The specific technique adopted depends on the properties of the food being preserved. For example, a traditional method for preserving rice is to allow it to dry naturally in the fields or on drying racks in barns for about two weeks. After this period of time, the native rice is threshed and then dried again by allowing it to sit on straw mats in the sun for about three days.

Modern drying techniques make use of fans and heaters in controlled environments. Such methods avoid the uncertainties that arise from leaving crops in the field to dry under natural conditions. Controlled temperature air drying is especially popular for the preservation of grains such as maize, barley, and bulgur.

Vacuum drying is a form of preservation in which a food is placed in a large container from which air is removed. Water vapor pressure within the food is greater than that outside of it, and water evaporates more quickly from the food than in a normal atmosphere. Vacuum drying is biologically desirable since some enzymes that cause oxidation of foods become active during normal air drying. These enzymes do not appear to be as active under vacuum drying conditions, however.

Two of the special advantages of vacuum drying is that the process is more efficient at removing water from a food product, and it takes place more quickly than air drying. In one study, for example, the drying time of a fish fillet was reduced from about 16 hours by air drying to six hours as a result of vacuum drying.

Coffee drinkers are familiar with the process of dehydration known as spray drying. In this process, a concentrated solution of coffee in water is sprayed though a disk with many small holes in it. The surface area of the original coffee grounds is increased many times, making dehydration of the dry product much more efficient.

Freeze-drying is a method of preservation that makes use of the physical principle known as sublimation. Sublimation is the process by which a solid passes directly to the gaseous phase without first melting. Freeze-drying is a desirable way of preserving food since it takes place at very low temperatures (commonly around 14°F to -13°F [-10°C to -25°C]) at which chemical reactions take place very slowly and pathogens survive only poorly. The food to be preserved by this method is first frozen and then placed into a vacuum chamber. Water in the food first freezes and then sublimes, leaving a moisture content in the final product of as low as 0.5%.

Salting

The precise mechanism by which salting preserves food is not entirely understood. It is known that salt binds with water molecules and thus acts as a dehydrating agent in foods. A high level of salinity may also impair the conditions under which pathogens can survive. In any case, the value of adding salt to foods for preservation has been well known for centuries.

Sugar appears to have effects similar to those of salt in preventing spoilage of food. The use of either compound (and of certain other natural materials) is known as curing. A desirable side effect of using salt or sugar as a food preservative is, of course, the pleasant flavor each compound adds to the final product.

Curing can be accomplished in a variety of ways. Meats can be submerged in a salt solution known as brine, for example, or the salt can be rubbed on the meat by hand. The injection of salt solutions into meats has also become popular. Food scientists have now learned that a number of factors relating to the food product and to the preservative conditions affect the efficiency of curing. Some of the food factors include the type of food being preserved, the fat content, and the size of treated pieces. Preservative factors include brine temperature and concentration and the presence of impurities.

Curing is used with certain fruits and vegetables, such as cabbage (in the making of sauerkraut), cucumbers (in the making of pickles), and olives. It is probably most popular, however, in the preservation of meats and fish. Honey-cured hams, bacon, and corned beef ("corn" is a term for a form of salt crystals) are common examples.

Freezing

Freezing is an effective form of food preservation because the pathogens that cause food spoilage are killed or do not grow very rapidly at reduced temperatures. The process is less effective in food preservation than are thermal techniques such as boiling because pathogens are more likely to be able to survive cold temperatures than hot temperatures. In fact, one of the problems surrounding the use of freezing as a method of food preservation is the danger that pathogens deactivated (but not killed) by the process will once again become active when the frozen food thaws.

A number of factors are involved in the selection of the best approach to the freezing of foods, including the temperature to be used, the rate at which freezing is to take place, and the actual method used to freeze the food. Because of differences in cellular composition, foods actually begin to freeze at different temperatures ranging from about 31°F (-0.6°C) for some kinds of fish to 19°F (-7°C) for some kinds of fruits.

The rate at which food is frozen is also a factor, primarily because of aesthetic reasons. The more slowly food is frozen, the larger the ice crystals that are formed. Large ice crystals have the tendency to cause rupture of cells and the destruction of texture in meats, fish, vegetables, and fruits. In order to deal with this problem, the technique of quick-freezing has been developed. In quick-freezing, a food is cooled to or below its freezing point as quickly as possible. The product thus obtained, when thawed, tends to have a firm, more natural texture than is the case with most slow-frozen foods.

About a half dozen methods for the freezing of foods have been developed. One, described as the plate, or contact, freezing technique, was invented by the American inventor Charles Birdseye in 1929. In this method, food to be frozen is placed on a refrigerated plate and cooled to a temperature less than its freezing point. Or, the food may be placed between two parallel refrigerated plates and frozen.

Another technique for freezing foods is by immersion in very cold liquids. At one time, sodium chloride brine solutions were widely used for this purpose. A 10% brine solution, for example, has a freezing point of about 21°F (-6°C), well within the desired freezing range for many foods. More recently, liquid nitrogen has been used for immersion freezing. The temperature of liquid nitrogen is about -320°F (-195.5°C), so that foods immersed in this substance freeze very quickly.

As with most methods of food preservation, freezing works better with some foods than with others. Fish, meat, poultry, and citrus fruit juices (such as frozen orange juice concentrate) are among the foods most commonly preserved by this method.

Fermentation

Fermentation is a naturally occurring chemical reaction by which a natural food is converted into another form by pathogens. It is a process in which food "goes bad," but results in the formation of an edible product. Perhaps the best example of such a food is cheese. Fresh milk does not remain in edible condition for a very long period of time. Its pH is such that harmful pathogens begin to grow in it very rapidly. Early humans discovered, however, that the spoilage of milk can be controlled in such a way as to produce a new product, cheese.

Bread is another food product made by the process of fermentation. Flour, water, sugar, milk, and other raw materials are mixed together with yeasts and then baked. The addition of yeasts brings about the fermentation of sugars present in the mixture, resulting in the formation of a product that will remain edible much longer than will the original raw materials used in the bread-making process.

Additional Topics

The vast majority of instances of food spoilage can be attributed to one of two major causes: (1) the attack by pathogens (disease-causing microorganisms) such as bacteria and molds, or (2) oxidation that causes the destruction of essential biochemical compounds and/or the destruction of plant and animal cells. The various methods that have been devised for preserving foods are all designed to red…

The term "thermal" refers to processes involving heat. Heating food is an effective way of preserving it because the great majority of harmful pathogens are killed at temperatures close to the boiling point of water. In this respect, heating foods is a form of food preservation comparable to that of freezing but much superior to it in its effectiveness. A preliminary step in many oth…

One of the most common methods for preserving foods today is to enclose them in a sterile container. The term "canning" refers to this method although the specific container can be glass, plastic, or some other material as well as a metal can, from which the procedure originally obtained its name. The basic principle behind canning is that a food is sterilized, usually by heating, an…

The majority of food preservation operations used today also employ some kind of chemical additive to reduce spoilage. Of the many dozens of chemical additives available, all are designed either to kill or retard the growth of pathogens or to prevent or retard chemical reactions that result in the oxidation of foods. Some familiar examples of the former class of food additives are sodium benzoate …

The lethal effects of radiation on pathogens has been known for many years. Since the 1950s, research in the United States has been directed at the use of this technique for preserving certain kinds of food. The radiation used for food preservation is normally gamma radiation from radioactive isotopes or machine-generated x rays or electron beams. One of the first applications of radiation for foo…

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Please I am need of both traditional and modern food preservation methods separately.

over 7 years ago

Where Boiling???? I need to make a scraps books :( help me..

almost 8 years ago

add

this doesnt have all the freaking info

almost 8 years ago

hello

Dude, this is boring.

over 7 years ago

A very good reminder on why hunger is becoming hazardous in Africa. We simply do not preserve food.

over 10 years ago

thanks for this information ,please if it possible send your references about freezing

almost 11 years ago

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almost 8 years ago

mohd hayat

after some time

almost 8 years ago

please write methods down but i was very useful otherwise and can you actually check the messages that people write on her it is very annoying :(

over 7 years ago

Dr.Ali Ahmed Elmardi

Please I am in need of the methods of preservation of meat ,milk,fish &camel milk.Iwant to know the permissable limit of feed aditive so as to elongate the life span of the product without putrifacton.

about 10 years ago

hi..thanks for the people who made this page...uhm..helped me with my HELE homework..by the way HELE stands for:~> Home~> Economics*and*~> Livelihood~> Education

over 1 year ago

tesfalem

food preservation

about 3 years ago

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over 3 years ago

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over 4 years ago

lee

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over 6 years ago

Md.Noyazzesh Hossain

Thank,s to you for providing infor mation.I want to know about the principle of freeze drying with other drying principle.

about 7 years ago

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about 7 years ago

AR.EYYY

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about 7 years ago

AR.EYYY

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about 7 years ago

John

Ok, for the person wanting to preserve milk, well besides the UHT method used which might be out of reach there was a patent and claim that a pyramid shaped milk container stopped milk from spoiling. So easy enough to try out, just a note that with pyramids the exact dimensions are critical so do your research and then make it with exacting accuracy to increase your chance of success.

yehey!i have my assignment now.Please I am in need of the methods of preservation of meat ,milk,fish &camel milk.Iwant to know the permissable limit of feed aditive so as to elongate the life span of the product without putrifacton.

over 7 years ago

lilian

in terms of project how do i start with the topic preservative methods of food

over 7 years ago

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over 7 years ago

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over 7 years ago

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almost 8 years ago

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can i ask something? what are the materials needed in fish preservation?

over 7 years ago

Please I am need of both traditional and modern food preservation methods separately.

almost 8 years ago

pooja

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almost 8 years ago

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about 10 years ago

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over 10 years ago

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over 11 years ago

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about 4 years ago

the monkeys arm pit

All this information is very interesting and valid i hope you will make many more but you need to add the author of the website or who writes it on this website and the date that it was last updated but other than that it was an amazing experience reading this and learning about all the preserved foods and how it works.Thank you so much

about 6 years ago

please i need a wide information on refrigeration of food items...i am doing a oral presentation this week friday.

Please tell me the suitable way of preservation of camel milk if the breeders are living far away from the large twons & the methods of tranportation to some extent difficlt?

almost 8 years ago

teoqua orr

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almost 11 years ago

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almost 7 years ago

the methods are not complete

almost 7 years ago

Thanks for the info.though sm msot interested in the principles underlying each met5hod of preservation

about 7 years ago

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about 7 years ago

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over 7 years ago

jayson

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almost 8 years ago

I like the content but would love to have more on how food intrinsic factors affect i.e types of microbes involved in food preservation techniques

almost 8 years ago

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about 7 years ago

Luckkkkynumba65

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over 7 years ago

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Can i please have you e mail adress so that i will ask for help during my research/dissertation.

please help me about the food preservation, i need notes about topic techniques, methodology.

7 months ago

I need notes on food preservation and processing

over 1 year ago

And also I have an assignment on role of processing and preservation animal product in food security,would you send source for doing of this assignment? Thanks for your help

over 1 year ago

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over 1 year ago

Seeking information on food preservation and packaging clases

about 3 years ago

Dear Sir.

aim professional Chef I like to start small tomato paste can packaging businesses at my home. I have to know Preservation method. I have small machine . at lest one Expires period. Plz send me same advice. which chemical I have to use for can packaging ..